The present invention relates to an optical pick-up apparatus, an apparatus for recording/reproducing information of an optical information recording medium, and beam expander, and particularly to an optical pick-up apparatus, objective lens and beam expander, by which variations of the spherical aberration can be effectively corrected in a high density optical information recording medium.
Recently, according to the practical use of a short wavelength red semiconductor laser, the development of a DVD (digital versatile disk) which is a high density optical disk whose size is the almost same as a conventional optical disk, that is, a CD (compact disk) which is an optical information recording medium, and whose capacity is greatly increased, is advanced, and in near future, it is presupposed that a higher density next generation optical disk also appears in the market. In the optical system of the optical information recording and reproducing apparatus using such the optical disk as a medium, in order to attain the high densification of the recording signal, or to reproduce the high density recording signal, it is required that a spot diameter to converge the light onto the recording medium through the objective lens, is reduced. In order to attain this requirement, there is the actual situation that the reduction of the wavelength of the laser as the light source or the increase of the NA of the objective lens are considered.
In this connection, when the reduction of the wavelength of the laser or the increase of the NA of the objective lens thus comes to be realized, even an almost negligible problem in the optical pick-up apparatus structured by the combination of the comparatively long wavelength laser and the objective lens of low NA by which the recording or reproducing of the information is conducted on the conventional optical disk such as the CD or DVD, it is more actualized.
A problem actualized in the combination of the shortening of the wavelength of the laser and the increase of the NA of the objective lens, is a variation of the spherical aberration of the optical system due to the temperature and humidity change. That is, in comparison with a glass lens, a generally used plastic lens in the optical pick-up apparatus is easily deformable due to the temperature or humidity change, and thereby, the refractive index changes. Even in a variation of the spherical aberration by the change of the refractive index which is not a problem in the optical system used in the conventional pick-up apparatus, its amount is not negligible in the combination the reduction of the wavelength of the laser and the increase of the NA of the objective lens, and a problem in which the spot diameter is increased, is generated. Accordingly, in the optical system employing a plastic lens, a spherical aberration becomes an important problem.
Further, another problem in the combination of the shortening of the wavelength of the laser and the increase of the NA of the objective lens, is deviation of a spherical aberration taking place on the objective lens due to the slight deviation of wavelength of the light source. In the semiconductor laser used as a light source in the optical pickup apparatus, there is a deviation of ±10 nm among actual products of the semiconductor laser. Therefore, if a semiconductor laser having an wavelength deviating from a reference wavelength is used as the light source, the spherical aberration taking place on the objective lens becomes larger as the numerical aperture becomes lager. Owing to this, if it is determined that the semiconductor laser having an wavelength deviating from a reference wavelength is used as the light source, the selection for the semiconductor laser to be used as the light source will be required. As a result, the cost of the semiconductor laser will be raised.
Further, another problem in the combination of the shortening of the wavelength of the laser and the increase of the NA of the objective lens, is deviation of a spherical aberration of the optical system due to errors in the thickness of a protective layer (or a transparent substrate) of the optical disk. Since the spherical aberration caused by the errors in the thickness of the protective layer occurs in proportion to fourth power of numerical aperture of the objective lens, the influence of the errors in the thickness of the protective layer becomes larger as the numerical aperture of the objective lens becomes larger, there my be a fear that recording or reproducing information can not be conducted stably.
In this connection, for the recording or reproducing of the information, between the optical disk of the next generation requiring the combination of the reduction of the wavelength of the laser and the increase of the NA of the objective lens, and the conventional optical disk, the wavelength of the light source and the NA of the objective lens are greatly different from each other as described above. Further, in order to suppress the coma greatly generated due to the tilt of the disk surface from the surface perpendicular to the optical axis which is presupposed in the optical disk of the next generation, it is effective to reduce the transparent substrate thickness, however, according to that, the transparent substrate thickness is greatly different from the conventional optical disk such as a CD. For example, an optical disk proposed for use in a next future generation comprises a transparent substrate having a thickness of 0.1 mm which is greatly different from the thickness of a transparent substrate of CD or DVD. Therefore, if the information of CD or DVD is reproduced by the objective lens for use in the next future generation, a large spherical aberration may be occurred. Accordingly, at least by using the common objective lens, without greatly increasing the cost, and by a compact optical pick-up apparatus, how to record or reproduce the information by suppressing the spherical aberration for the different optical information recording medium including the next generation optical disk, is a problem.
Further, the other problems is a problem of the axial chromatic aberration caused in the objective lens due to minute variations of the wavelength of the laser light source. The change of the reflective index due to the minute variation of the wavelength of the general optical lens material is larger as the short wavelength is used. Therefore, the defocus amount of the focal point caused due to the minute variation of the wavelength becomes large. However, as it can be seen from a fact that the depth of focus of the objective lens is expressed by k·λ/NA2 (k is a proportional constant, λ is the wavelength, and NA is a numerical aperture of an image side of the objective lens), the shorter the wavelength of the used light source is, the smaller the depth of the focus is, and even a few defocus amount is not allowed. Accordingly, in an optical system using a short wavelength light source such as the blue purple semiconductor laser (about 400 nm wavelength) and an objective lens having a high image side numerical aperture, in order to prevent a wavelength variation due to the mode hop phenomenon of the semiconductor laser, or the deterioration of the wave front aberration due to the high frequency superimposition, a correction of the axial chromatic aberration becomes important.